Scientists said Tuesday that they are expecting smaller-than-average hypoxic levels this year, meaning a smaller-than-average “dead zone” in the nation’s largest estuary.

Hypoxic zones with very low oxygen and anoxic zones with no oxygen are caused by excessive nutrient pollution from fertilizer runoff from farm fields and lawns.

Nutrients like nitrogen and phosphorous can spur algae blooms in the water, consuming oxygen needed by marine life. Weather conditions, including wind speed, wind direction, precipitation and temperature, also play a role in dead zones.

Researchers say the Chesapeake dead zones, which have varied considerably in recent years, threaten an effort to restore the Bay’s water quality and enhance its production of crabs, oysters, and other fisheries.